Quality paper is commonly produced on multi-stage paper making assemblies. Generally, a paper making assembly includes a forming stage, a press stage, a dryer stage, and a winding and cutting stage. Several of these stages incorporate an endless loop or web (e.g., fabric or felt endless web) on which the paper may be formed or treated.
The paper making process begins by placing a slurry mixture on a first web, employed by the forming stage machine. Next, in the press stage, as water is removed, the paper mixture begins to take shape on a second web. During these early stages, the paper's surface characteristics are dictated by the web surfaces on which the paper is being formed. That is, the finished paper will include imperfections as a result of the webs embedding their imperfections into the paper. Thus, to avoid these imperfections, the webs must be manufactured to precise specifications, maintaining a surface that is smooth and devoid of any imperfections or variations. Ideally, the webs are made of endless loops, i.e., without seams, which helps to minimize the imperfections.
Typically, endless webs are first manufactured on web making machines, such as large needling looms. One such needling loom is available from the assignee of the present invention, namely, Morrison Berkshires, Inc., of North Adams, Mass. Then, the webs are transferred to a finishing assembly for setting. There, the webs are typically treated with chemicals, mechanical and thermal energy, and the like. The webs also may be stretched while otherwise being treated to achieve a further smooth set surface. The setting process performed on a finishing assembly also must be performed with care and to strict specifications in order to avoid imparting unacceptable imperfections to the webs.
Certain web finishing assemblies include: at least one inside treatment roll, which may heat the endless web on contact (i.e., head roll); at least one inside treatment roll, which may stretch the endless web; and possibly a non-contacting treatment device, such as an air box set (or oven), for heat-setting the endless web. Certain finishing assemblies, used in connection with the present invention, are described in commonly assigned U.S. Pat. Nos. 5,901,422 and 5,901,423, both of which are incorporated by reference herein.
In addition to the stretch roll, the head roll and the air box set, a press roll may be employed. A press roll is typically operated adjacent to one of the inside treatment rolls, for instance, the head roll. Whereas, during operation, the head roll is arranged to contact the inside of the web, the press roll is usually arranged to contact the outside.
The press roll is typically used in combination with the head roll to impart a nip pressure to the fabric web. The press roll also may be used to impart mechanical properties to the web such as certain caliper (i.e. thickness), pre-compaction, and the like. This, in turn, will aid in the removal of working fluid from the web. That is, as the fabric web is being set, certain liquid chemicals may be applied to it. To remove this excess working fluid, the web is nipped between the press roll and the head roll. As a result, the working fluid is pressed out. The working fluid also may drop out on its own due to the web being saturated.
Known press roll assemblies include a press roll adjacent the head roll, spaced apart to allow the fabric web to travel therebetween. In addition, backup rollers may be employed adjacent the press roll, and generally opposite to the head roll. The backup rollers are generally used to support the press roll and prevent it from moving in the machine direction from its predetermined position in relation to the head roll. Additionally, force actuators, typically coupled to pairs of backup rollers, are provided to impart a pressing force to the press roll, through the backup rollers, to create the desired nip pressure between the press roll and the head roll.
In this type of press roll arrangement, it is desirable to apply a consistent nip pressure throughout the entire length of the press and head rolls, at least where the web is located. This will provide for a uniformly set fabric web having a generally uniform caliper. It also is desirable to generate a maximum nip pressure, to remove as much working fluid as possible and impart mechanical and performance properties to the web. This is commonly achieved by using a small diameter press roll.
Small diameter press rolls are preferred because nip pressure is inversely proportional to the diameter of the press roll, at any given force. It should be noted that the larger the force necessary to obtain the required nip pressure, the larger and stronger the head roll so as to keep the overall deflection of the head roll down to a minimum.
Typically, small diameter press rolls will deflect under a normal load and will highly deflect under relatively high loads. It is desirable to keep the amount and degree of deflection of both the press roll and the head roll the same to provide for a uniform pressing load and assure a uniform nip area and thus a generally uniform caliper of the web. Ultimately, it is desirable to have the press roll's deflection characteristics follow those of the head roll.
There are certain problems associated with known press roll systems. One is the non-uniform deflection of the relatively smaller press rolls as compared to the deflection of the head roll. This results, in part, from the press roll not being evenly supported by the force actuators (or actuating devices), via the backup rollers. Instead, discrete pressure points are translated from the force actuators, through the backup rollers, to the press roll. As a result, inconsistencies are imparted to the nip area, hence, causing variations in the web, such as an irregular caliper throughout the web.
Another problem with existing press roll systems is the inconsistency imparted to the web from the arrangement of the backup roller. That is, the backup rollers are normally paired or grouped along the cross machine direction of the web. As a result, gaps exist where no backup rollers are in contact with the press roll. This arrangement imparts an uneven pressure at various points along the press roll, which, in turn, causes variations in the web. That is, in known press roll systems, typically pairs of backup rollers are each coupled to the pair's own force actuator(s). The backup rollers are placed under the press roll in the cross machine direction of the web. Gaps between each pair or group of backup rollers sometimes causes the force upon the press roll to be inconsistent along the cross machine direction of the web. This causes inconsistencies such as fabric markings, variations in the caliper of the web and other such inconsistencies on the web.
Finally, after the working fluid has dropped out or has been pressed out of the web, it typically drains onto the force actuators and other ancillary machinery arranged under the press roll and backup rollers of the finishing assembly This may lead to contamination of those parts.
Thus, there is a need for a press roll system having a relatively small diameter press roll, which is capable of deflecting in step with the deflection of the head roll. There also is a need to have an arrangement of backup rollers that is independent of, or not associated with, any particular force actuator. This arrangement would minimize inconsistent application of forces to the press roll. Finally, there is a need for a press roll system that is capable of channeling the working fluid away from the force actuators and other machinery associated with the press roll system and finishing assembly.